Note: Descriptions are shown in the official language in which they were submitted.
W O 91/1743() r~ ~ PC~r/US91/03039
80LID ~T~TE ION SENSOR ~IT~ ~ILICONE ~E~LBRUUNE
Background of the Invention
This invention relates generally to devices and
systems for measuring concentrations of ions, chemicals,
biological materials, and reaction products, and more
particularly, to a solid state device which employs a
silicone rubber matrix, having electrochemical properties
comparable in quality to conventional PVC membranes, as the
substance-sensitive membrane, and wherein the silicone
rubber membrane exhibits excellent adhesion to the Si3N4
surface of solid state sensors.
As a result of their small size and potentially lower
cost, solid state ion sensors are of interest in industrial
and medical applications as replacements for traditional
ion-selective electrodes. These sensors make possible new
direct-monitoring applications. There is a need, however,
for improved membrane adhesion, as such would be benefi-
cial, not only to all users of solid-state chemical
sensors, but particularly those interested in long-term
monitoring.
Basically, the potential uses of solid-state ion
sensors can be divided into industrial and medical applica-
tions. Industrial uses include, for example, the monitor-
ing of treated or waste water for hardness or pollutants;
on-line analysis of industrial chemicals, foodstuffs, and
medicines; and low cost analytical instruments. Medical
applications include the monitoring of electrolytes, blood
gases, and metabolic substrates, both for biochemical
control systems and for patient monitoring or diagnostics.
There is a need for solid state ion sensors which can
achieve the needs of industrial and medical monitoring for
very long periods of time. Silicone-based chemical sensors
often use ionophore-doped polymeric membranes as transduc-
ers because of their excellent selectivity toward the ion
of interest, the wide range of ions for which ionophores
are available, and because they can borrow from ongoing
W O 91/1743~ PC~r/US91/03039
developments in ion-selective electrode technology. As
is the case with ion-selective electrodes, most solid state
sensors have used poly(vinylchloride) (PVC) as a membrane
matrix. One of the primary causes of failure in conven-
tional microsensors has been poor adhesion of the organicmembrane to the chip surface. This leads to the formation
of electrolyte shunts around the membrane, rendering the
membrane inoperative.
Others in the prior art have endeavored to improve
membrane adhesion such as by the use of a polyimide
suspended mesh, modification of PVC for binding to hydrox-
yl-bearing surfaces, and mechanical attachment of the
membrane. These methods have tended to improve adhesion of
the membrane, but generally have resulted in either
inferior electrochemical performance when compared to PVC
or added processing complexity. There is, therefore, a
need for a permselective membrane which exhibits good
electrochemical properties, preferably at least as good as
traditional PVC membranes, but which exhibits excellent
adhesion to the Si3N4 surface of solid state sensors.
Silicone has been used for forming chemical-selective
membranes. Usually, such membranes are in the form of a
silicone rubber tubing which is impregnated with an
ionophore. In other known arrangements, the silicone
rubber is pressed into a pellet. One commercially solvent
castable two-part system which is based on silicone rubber
has been employed to form chemical-selective membranes.
However, the only success with this known approach has been
in sensing pH. Moreover, the known approach produced a
membrane which did not adhere well to surfaces.
It is, therefore, an object of this invention to
provide a substance-sensitive solid state sensor which has
an extended lifetime.
It is another object of this invention to provide a
substance-sensitive membrane system for a solid state
sensor which is possessed of excellent electrochemical
properties.
WO91/17430 PCT/US91/03039
3 2 i) v ~ ~ 1 P~
It is also an object of this invention to provide a
substance-sensitive membrane system for a solid state
sensor which is characterized with excellent adherence to
solid state sensor materials.
It is a further object of this invention to provide a
substance-sensitive membrane system for a solid state
sensor which can be applied to a plurality of solid state
devices simultaneously using conventional integrated
circuit manufacturing techniques.
It is additionally an object of this invention to
provide a solid state sensor system which is not subject to
the generation of disabling electrolyte shunts around the
substance-sensitive membrane.
It is yet a further object of this invention to
provide a solid state sensor system which is simple and low
in cost.
It is also another object of this invention to provide
a substance-sensitive polymeric membrane system for a solid
state sensor which can be applied to a multiplicity of
solid state devices simultaneously using conventional
integrated circuit manufacturing techniques and which
utilizes ionophoric doping to create the substance sensi-
tivity.
It is yet an additional object of this invention to
provide a substance-sensitive membrane for use with a solid
state sensor and which does not require a structural layer
associated therewith to maintain communication between the
membrane and a solid state substrate.
It is still another object of this invention to
provide a substance-sensitive solid state sensor which can
be manufactured inexpensively in production quantities, and
which can be adapted for industrial uses, such as monitor-
ing treated or waste water for hardness or pollutants, on-
line analysis of industrial chemicals, foodstuffs, and
medicines, and low cost analytical instruments.
It is a yet further object of this invention to
provide a substance-sensitive solid state sensor which can
WO91/~743(~ PCT/~S91/03039
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~ ; - 4
be manufactured inexpensively in production quantities, and
which can be adapted for medical uses, such as monitoring
of electrolytes, blood gases, and medical substrates.
It is also a further object of this invention to
provide a substance-sensitive solid state sensor which can
be manufactured inexpensively in production quantities, and
which can be adapted for biochemical control systems.
It is additionally another object of this invention to
provide a substance-sensitive solid state sensor which can
be manufactured inexpensively in production quantities, and
which can be adapted for patient monitoring and diagnos-
tics.
A still further object of this invention is to provide
a substance-sensitive membrane for use in a solid state
sensor, wherein the membrane exhibits good adhesion to sio2
surfaces.
An additional object of this invention is to provide
a substance-sensitive membrane for use in a solid state
sensor, wherein the membrane exhibits good adhesion to Si3N4
surfaces.
Yet another object of this invention is to provide a
substance-sensitive membrane for use in a solid state
sensor, wherein the membrane exhibits a reduced electrical
resistance characteristic and thereby yields reduced
electrical noise for the sensor.
Another object of this invention is to provide a sub-
stance-sensitive membrane for use in a solid state sensor,
wherein the membrane exhibits a reduced tendency to adsorb
protein.
A yet further object of this invention is to provide
a substance-sensitive membrane for use in a solid state
sensor, wherein the membrane exhibits a reduced tendency to
cause blood clotting.
Summary of the Invention
The foregoing and other objects are achieved by this
invention which provides an integrated circuit chemical
W09l/l~43(J ~ J 1 1 PCT/US91/03039
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sensor arrangement. The integrated circuit sensor is
provided with an input electrode formed of a conductive
material in the vicinity of a region formed of a silicon-
based semiconductor material, and a permselective membrane
having a predetermined electrochemical property and formed
of a silicone-based compound. The silicone-based compound
is arranged to be in adherence with the silicon-based
semiconductor material, and in electrical communication
with the input electrode. In this manner, a voltage
responsive to the electrochemical property is produced at
the input electrode.
In a preferred embodiment of the invention, the
permselective membrane is formed of polydimethylsiloxane,
which is commercially available (Petrarch-SS). A resis-
tance-reducing additive, such as CN-derivatized silicone
rubber is mixed into the polydimethylsiloxane, which serves
to reduce electrical noise associated with a high impedance
characteristic of the polydimethylsiloxane. In addition,
an ionophore is mixed with the polydimethylsiloxane, which
ionophore may be, for example, potassium or ammonium. In
other embodiments, the sensors be made responsive to other
molecules through addition of bioactive agents, such as an
enzyme, an immunochemical, a bacteria, and a virus.
In accordance with a process aspect of the invention,
a substance-sensitive membrane is formed by the process of
first mixing an ionophore into polydimethylsiloxane, and
second mixing CN-derivatized silicone rubber into said
dissolved polydimethylsiloxane.
In one embodiment of this process aspect of the
invention, there is provided the further step of depositing
the mixture of the polydimethylsiloxane, the ionophore, and
the CN-derivatized silicone rubber onto a substrate. In
other embodiments, the mixture may be solvent cast.
As previously noted, the ionophore may be, for
example, an ammonium ionophore or a potassium ionophore.
Illustratively, the ionophore is present in a proportion of
1 wt.% of the mixture. The CN-derivatized silicone rubber
~'0 91/17430 PC~r/US91/03039
i, 6
is present in the mixture in an amount of approximately 10
wt.%.
Brief Description of the Drawin~
Comprehension of the invention is facilitated by
reading the following detailed description, in conjunction
with the annexed drawing, which is a graphical representa-
tion illustrating the response of four different types of
membranes to potassium, including a measurement in blood
serum.
10 Detailed Descri~tion
The present invention has been made by the inventors
herein in the form of silicone-matrix membranes selective
to potassium and ammonium. Such membranes exhibit good
electrochemical properties and superb adhesion to silicon
nitride. The highly beneficial adhesion characteristic was
accomplished by using polydimethylsiloxane which advanta-
geously is easily cured at room temperature by moisture
activation. The polydimethylsiloxane (Petrarch-SS) is
silanol-terminated and can be processed in the solvent
tetrahydrofuran (THF). This silicone material exhibits
extremely high bond strength to a wide range of substrates,
including Si3N~. In addition, the polydimethylsiloxane
exhibits reduced protein adsorption and blood clotting.
The foregoing notwithstanding, it is a problem with
silicone membranes in general that they are characterized
by high electrical resistance. This causes compromises in
the electrochemical performance, illustratively in the form
of signal noise. The signal noise associated with the high
resistance characteristic can be improved by incorporating
a plasticizer into the membrane, or one of the lipophilic
additives, such as potassium tetrakis-para-chlorophenyl
borate, or cyano- group derivatized silicone rubber.
Unfortunately, the use of a plasticizer causes poor
adhesion. Of the other additives, CN-derivatized silicone
rubber gives the best noise reduction. Accordingly, 10% by
W O 91/17~30 PC~r/US91/03039
r
7 ~ 3
weight of this material is added to the polymer, along with
1% by weight ionophore.
The following table compares the electrochemical
properties of four different membrane types toward potassi-
S um and ammonium. Although the silicone membrane appears tohave a slightly inferior performance, its greatly superior
adhesion characteristic will justify its use in many
applications.
TABLE 1
r~
Cw~ on Slopc Da~ Sclcaivity ~ R A
of M-tric (mV/d~dc) l~mit ~ ' i=NH,+
(~vtX) K~ NH,- lC' NH.- j=N~- j=N~' j=lC'
PVC (33X) 57,1 55.8 0.5 0.98 4.28 -2.82 4.85 1.0 P
PVC ao%) 57.3 56.0 0.52 0.91 4.22 -2.T7 -0.82 0.8 G
pVClAclAI
aO%)
PU a6.4x) 57.2 56.8 0.59 0.99 4.21 -2.89 4.89 0.9 E
PVCIAclAI
2 0 (6.6%)
Y-SS (78X) 56.5 52.9 1.0 1.4 4.16 2.64 -0.66 33.5 S
9~-CN al X)
pVC pot~virJgl rJc R N~m~ od Rc i~cc
PVC/ArlAI 80X virlgl c~bride
2 5 5 % viogl ~c A Q~ tr~c Adhcucn
15% vingl ~d P Poor
PU pdgwb~c a aOOd
P-SS pdg~ndbglriloYnc 1~ ~cclleot
Si-(N 10~12% (cg~pgl)~dhgl S Sq~rb
'Pr~ od ~ium ~bvc a~nb~ rc dopcd ~nh 1% v~cio ~ r~uctm, rc poctivclg. ~c b~lrncc of îhc mcr~br Dc
i~ DOA (birQ ahglhc~gl)-dipuo plrAbi~:r. (Tbc ilic~o rubbcr rlaDbnnc coo~u ro p~ zcr~)
In the figure, a~graphical representation illustrates
the response of the four different types of membranes to
potassium, including a measurement in blood serum. It is
evident from this figure that the silicone membrane is
eminently usable in a wide variety of monitoring situa-
tions.
Membrane adhesion is significant because it is one of
the properties which determines the lifetime of solid state
chemical sensors. In order to determine the adhesion
characteristic during testing of the membranes of the
present invention, and others, the well-known "Scotch tape
WO91/17430 ~ ^ PCT/US91/03039
Q~ -
test" was employed. The results of this testing process is
shown below in tabular form in Table 2.
TABLE 2
A D H E S I O N T E S T S
Num~er of Membrane~ R~moved
Scotch-Tap* Peel Te~t SoaX Te~t
Matrix Type 30 Spots on Si3N, 24 Gla~ Tube3
No SiCl4 With SiCl4 6-hour Soak
PVC 30 30 19
PVC/(PVC/AcJAl) 30 30 no~e
PU/(PVC/A~/Al) none none none
Silicone none ____ none
As set forth in Table 2, thirty membranes, each of
four different matrices, were cast on silicon wafers upon
which a Si3N4 film had been deposited. After the membranes
were cured, the tape was pressed onto the membranes and
then removed. None of the polyurethane or silicone rubber
membranes came off, while all of the PVC and hydroxylated
PVC membranes were removed.
In another experiment, wet adhesion was tested by
coating glass tubes with each composition and soaking the
tubes in water for six hours. When the tubes were rinsed
with flowing water, many of the PVC coatings washed away
from the glass tubes, but such was not the case for the
polyurethane or silicone-based membranes.
In view of the fact that the foregoing tests are only
semi-quantitative, a new adhesion test was designed. The
membranes were cast on a Si3N~ surface of a wafer which had
been scribed on the backside. The wafer was then cleaved,
and mounted on a pull tester with one half attached to a
load cell, and the other half to a stationary grip. The
wafer halves were pulled apart, peeling the membrane from
the surfaces, while the membrane peeling force was moni-
tored by the load cell. With this test, the maximum
WO91/17~3~ PCT/~S91/03039
pulling force required to peel each membrane from the Si3N4
surfaces was compared. These results are set forth below
in Table 3.
Table 3
A D H E S I O N T E S T R E S U L T S
Normalized Pulling Force
Matrix Type Dry ¦Wet
.
no SiCl4 with SiCl4 with SiCl4
15 PVC l.00 1.49 l.Ol
PVC/(PVC/Ac/Al)0.54 l.04 l.Ol
PU/(PVC/Ac/Al)5.29 54.99 30.84
20 Silicone 57.87 _____ _____
As is evident from Table 3, the silicone rubber
membrane had extremely strong bond strength. In fact, the
bond strength of the silicone rubber membrane was so great
that it exhibited cohesive failure rather than peeling.
The membranes pulled apart at about S0 times the force
required to peel the PVC membranes, but adhesive strength
was higher than that since the membranes failed cohesively
rather than adhesively. Thus, the adhesive strength of the
membranes made in accordance with the present invention is
greater than that shown in the table, and clearly greater
than the adhesive strength of the other membranes. As
expected, the adhesion of most of the membranes was
decreased after soaking in water for 24 hours, but no
change was seen with the silicone membrane.
There are many ways by which the membrane could be
applied, including dipping, casting, spin-coating, screen
printing, etc. Moreover, innumerable slight variations in
the formulas could result in useful membranes. The
simplest membranes are those containing ionophore to a
W091/17~30 ~ PCT/US91/03039
~ ' v '~
specific ion. Such membranes should also be used in
conjunction with, or incorporate in, the membrane matrix,
a bioactive agent, such as an enzyme, an immunochemical, a
bacteria, etc. Thus, the membranes can be made specific to
more complex chemicals.
Although the invention has been described in terms of
specific embodiments and applications, persons skilled in
the art can, in light of this teaching, generate additional
embodiments without exceeding the scope or depa~ting from
the spirit of the claimed invention. Accordingly, it is to
be understood that the drawing and description in this
disclosure are proffered to facilitate comprehension of the
invention, and should not be construed to limit the scope
thereof.
